1. Field of the Invention
This invention relates to a method for evaluating bonding properties of a metallic pipe. More particularly, this invention relates to a method for non-destructively measuring a step fault and a defect produced at the bonding portion of a metallic pipe such as plant piping, a line pipe and an oil well pipe or the like, or for measuring a thickness of the bonding portion in order to evaluate, in advance of carrying out an pipe expansion operation to expand its diameter, if there is a possibility that a crack or the like might be produced at the bonding portion of the metallic pipe upon the pipe expansion operation.
2. Description of Related Art
Conventionally, in the technical fields such as a chemical industry and a petrochemical industry, some long metallic bonded pipes such as plant piping, a line pipe and an oil well pipe have been used until now in order to transport corrosive fluid over a long distance. For example, the pipe line is used for transporting crude oil obtained from an oil reservoir to an oil refining plant and the like and its length reaches a length in excess of over several tens of kilo-meters.
In addition, upon drilling an oil well, a steel pipe called a casing tube is buried in a down-hole to protect the down-hole drilled into the ground or prevent leakage of crude oil. Usually, the oil reservoir is at several thousand meters under the ground, so that it is necessary for the casing tube having a length of several thousand meters to reach the oil reservoir.
In turn, a seamless steel pipe having a superior corrosion resistance property is usually applied in a corrosive environment. However, a length of such a seamless steel pipe which is industrially produced in volume is about 10 to 15 m and the maximum limit of the length of such pipes possibly manufactured is about 100 m or so. Accordingly, the long metallic pipe such as a line pipe or an oil well pipe or the like is usually manufactured by joining a plurality of relatively short seamless steel pipes having a length of about 10 to 15 m or so.
As a method for joining metallic pipes to be applied for such applications as above, a screw joining method (a mechanical coupling method), a welding method (an orbital welding method), a friction welding method and a diffusion bonding method and the like are well known.
The screw joining method is a method in which metallic pipes are bonded together by screwing the end parts of the metallic pipes. The welding method is a method in which the metallic pipes provided with a groove at the end surface of each pipe are abutted against each other, and molten metal is filled in the grooves so as to bond the metallic pipes together.
On the other hand, as the diffusion bonding method, there are provided a solid phase diffusion bonding method and a liquid phase diffusion bonding method. In the diffusion bonding method, two metallic pipes are abutted directly to each other and elements are diffused while keeping a solid phases therebetween. In the liquid phase diffusion bonding method, an insert material is inserted into the bonding interface between the two metallic pipes, and then the insert material is melted to diffuse some of the elements at the metallic pipe.
The diffusion bonding method has some advantages that a joint obtained by this method has a superior property in strength and air-tightness compared to that of the screw joining method. Due to this reason, the diffusion bonding method is widely used as a method for connecting the metallic pipes such as an oil well pipe or line pipe and the like.
However, despite the advantage as described above that a high quality joint can be attained, the diffusion bonding method also has some disadvantages that bonding properties such as strength, toughness and the like greatly vary due to various kinds of defects produced at the bonding interface and the causes are found in a wide range. Especially, in the case where some defects such as cracks and poor bonding are produced at the edge of the bonding interface, they may cause a remarkable reduction in strength and fatigue properties of the bonded body due to a notch effect.
For example, there may be a case where the pipes are bonded together with the axes of the metallic pipes being inevitably misaligned from each other by the diffusion bonding method. In that case, a step fault is produced at the outer circumferential surface and/or inner circumferential surface of the bonding portion.
In addition, the metallic pipe industrially manufactured under in volume are usually with a predetermined size tolerance, and therefore there are variations in an outer diameter and a wall thickness of those metallic pipes are distributed within the range of the size tolerance. Due to this fact, even if the metallic pipes are bonded together under a state where their axes coincide with each other, a certain step fault is produced at the outer circumferential surface and/or the inner circumferential surface of the bonding portion.
In particular, in the case of bonding the metallic pipes by the liquid phase diffusion bonding method, the molten insert material may be squeezed out of the bonding portion and solidifies to produce a step fault there.
The step fault produced at the outer circumferential surface and/or inner circumferential surface of the bonding portion may be subjected to stress concentration which inevitably causes a connecting strength a fatigue properties to decline. For example, upon a pipe expansion operation, a pipe expansion tool is inserted into the metallic pipe and passes through the bonding portion in order to cause the plastic deformation there. At the time, a strong stress is applied to the bonding portion of the metallic pipes. Due to the strong stress, if a step fault is present at the inner circumferential surface of the bonding portion, the step fault may be subjected to a stress concentration which may cause to produce a crack at the bonding portion. In addition, when a step fault is produced at the inner circumferential surface of the bonding portion, corrosive substances may easily be accumulated there. As the result, the corrosion resistance and mechanical properties may be adversely influenced.
Also, in the case where a shape defect such as a crack is already present at the bonding portion, stress concentration occurs at the defect part, which may lead to produce a crack upon a pipe expansion operation. This is also true in the case where a metallic structural defect such as a poor diffusion of molten insert material at the base member is present, given that the aforesaid liquid phase diffusion bonding method is applied.
As to the step fault produced at the outer circumferential surface of the bonding interface, it can be easily detected by a visual inspection. Also, the defect produced at the outer circumferential surface of the bonding interface may be detected relatively easily by various kinds of non-destructive examinations such as an ultrasonic examination, a magnetic particle examination and a liquid penetrant examination or the like.
However, a step fault produced at the inner circumferential surface of the bonding interface is extremely difficult to be detected by a visual inspection. Further, there is no prior art to suggest or propose a method for detecting a step fault produced at the inner circumferential surfaces of the metallic pipes bonded by diffusion bonding without destruction of the pipes to be examined.
In addition, although the defect produced at the inner circumferential surface of the bonding interface may possibly be detected through various kinds of non-destructive examination method, there is no prior art to suggest an examination method to discriminate a step fault and a defect are reliably discriminated in the case where both step fault and defect are produced at the inner circumferential surface of the bonding interface, as well as to detect sizes of the step fault and defect with high accuracy.
Additionally, in the field of an oil well pipe, for example, in order to reduce a cost of drilling an oil well and to increase a production efficiency, an attempt has been made to expand a bonded body having a small diameter an inner diameter with the use of a mandrel or the like after it is buried in a drilled down-hole. However, if there is a step fault at the inner circumferential surface of the bonding portion, a stress is possibly concentrated at the step fault upon expanding the inner diameter by the mandrel or the like, which possibly produce a crack at the bonding portion.
In addition, there may be the cases where the crystal structure at the bonding portion undergoes increase in the grain size of the particles due to excessively high heating temperature during a diffusion bonding or where a diffusion bonding is insufficient without the occurrence of grain-coarsening due to an excessively low heating temperature or insufficient heating. In these cases, there is a possibility that a crack is produced at the grain boundaries or separation at the bonding portion of the metallic pipes upon pipe expansion.
If a plant or the like is assembled with the metallic pipes having a crack or a separation at the bonding portion, which have been over looked, production troubles at a site are inevitable. These troubles include that leakage of crude oil when a petroleum drilling is carried out in the case of applying such pipes as oil well pipes, or a gas leakage in the case of applying such pipes as pipe lines.
In addition, to replace the metallic pipe with a new metallic pipe or to repair a crack upon occurrences of a deficiency at the bonding portion of the metallic pipes, it requires tremendous amount of labor and expenditure for the repairing work.
Accordingly, in order to assure reliability in the resulting bonded properties of metallic pipes, it is important to measure a size of the step fault produced at the bonding portion quantitatively after bonding operation. In the prior art, such a measurement as described above has been carried out only on the outer surface of the bonding portion for the step fault produced outside with the use of slide calipers.
However, such a measurement of the step fault with the use of slide calipers has a problem that a large error may be produced if a size accuracy or surface flatness is poor as in the case of a seamless steel pipe. In addition, there is also a problem in the case where a step fault present at the bonding portion is wide, the number of measuring points must be increased and a large amount of measuring time is required to improve the accuracy of the measurement. Further, it is difficult to measure a step fault produced at the inner circumferential surface of the bonding portion with the slide calipers. In sort, there remains a problem that there is no means for performing an measurement of a step fault produced at the inner circumferential surface accurately and efficiently.